Underground formation waters can contain resident ions such as barium (e.g. at a level of up to 3000 ppm, for example 50-500 ppm) and usually also calcium (e.g. at a level of up to 30,000 ppm, for example 1000-5000 ppm) both in the form of soluble chlorides, but also in the presence of sulphate ions, so the water is saturated with barium sulphate, and usually also calcium sulphate. This formation water can meet seawater water, which can contain precipitate precursor ions such as soluble carbonate (e.g. at 100-5000 ppm) and sulphate (e.g. at 1000-3500 ppm). Mixing the two waters produces an aqueous supersaturated solution of barium sulphate and/or barium carbonate, and/or calcium sulphate and/or calcium carbonate, from which scale comprising these compounds deposits on surfaces. The meeting of the two waters can be in the formation, when seawater containing precipitate precursor ions is injected into the formation through an injection well at a distance from a production well to enhance oil recovery (i.e. a water flood treatment). The scaling may occur in the production well or downstream thereof e.g. in flow lines, or gas/liquid separators (for separating oil/water from gas) or in transportation pipelines leaving the gas/liquid separators. Carbonate scale may particularly form in the gas/liquid separator or downstream thereof, due to reduction in gas pressure causing soluble calcium bicarbonate to form insoluble calcium carbonate.
Scaling problems may be mitigated by using scale inhibitors. Traditionally, scale inhibitors have been squeezed into a formation from a production well. However, it has recently been proposed to introduce scale inhibitors into a formation via an injection well. Thus, WO 03/106810 describes a method of inhibiting scale formation in a subterranean formation comprising:
(a) injecting a suspension comprising particles of a controlled release scale inhibitor suspended in an aqueous medium into a formation through an injection well wherein the particles have a mean diameter of less than 10 microns, preferably less than 5 microns, more preferably less than 1 micron;(b) allowing the suspension to percolate through the subterranean formation towards a production well; and(c) controllably releasing the scale inhibitor from the particles in the near well bore region of the production well.
A method of controlling deposition of mineral scale from an aqueous supersaturated solution of the mineral salt onto a solid surface is described in WO 00/79095. This method does not involve the use of a scale inhibitor compound. Instead the method relies on:    (a) forming a composition comprising a dispersion of either (i) seed crystals of the mineral salt in an aqueous solution of the mineral salt or (ii) seed crystals of a salt isomorphous with the mineral salt in an aqueous solution of the isomorphous salt, the seed crystals of the dispersion having a Mean particle size of less than 2.5 microns;    (b) distributing said composition into either (i) an aqueous supersaturated solution of the mineral salt or (ii) an aqueous precursor liquid of the aqueous supersaturated solution which aqueous precursor liquid is saturated with respect to the seeds, and in the case of (b)(ii) converting the aqueous precursor liquid into an aqueous supersaturated solution of the mineral salt; and    (c) contacting the treated aqueous supersaturated solution with the solid surface.
The dispersion of seed crystals is preferably formed by subjecting an aqueous supersaturated solution of the mineral salt or of the isomorphous salt to sonic or ultrasonic vibration. WO 00/79095 describes applying the treatment when two or more formation waters or production waters meet, or either or both meet seawater. Such mixings occur when the waters especially production waters from more than one source are to be reinjected downhole via a single line. The treatment is especially applied between a produced water separator and a commingling location. However, it is clear that this treatment is only intended to prevent deposition of scale on metal surfaces in the injection well and that it was not envisaged that seed particles could propagate through the formation together with the injection water.